Chemical Inhomogeneity Populations in Various Zircaloy Claddings and Their Association with SCC and Corrosion Resistance

A technique has been developed that permits detection and characterization of sparsely distributed chemical inhomogeneities in Zircaloy. These inhomogeneities have previously been observed at the origins of iodine stress-corrosion cracks but are not detectable by, for example, simple scanning electron microscopy (SEM) examination. The technique uses radioactive iodine to “label” the chemical inhomogeneities, autoradiography to detect their locations, and SEM and energy-dispersive X-ray analysis (EDAX) to further characterize them. Large areas of surface have been surveyed and statistically meaningful populations of chemical inhomogeneities measured for five different lots of Zircaloy cladding. Inner surfaces and cladding cross-sectional surfaces have been studied. There are clear differences in chemical inhomogeneity size distribution and composition between the various claddings.

For three of the claddings characterized in this work, the previously measured stress-corrosion cracking (SCC) threshold stresses correlate well (inversely) with the new data on their average chemical inhomogeneity sizes. Of special interest is the fact that the most SCC-resistant cladding contains far fewer iron-bearing inhomogeneities than the other claddings.

To investigate whether chemical inhomogeneities relate to the aqueous corrosion performance of the material, a blind test was conducted on 14 samples representative of material previously tested in an autoclave at 500°C at 10 342 kPa (1500 psi) for 24 h. The unautoclaved companion samples were examined by autoradiography. The results show that, in general, materials with good corrosion performance in the high-temperature autoclave tests are also characterized by a low population of chemical inhomogeneities and vice versa.

Selected chemical inhomogeneities were cross-sectioned and studied further using the electron microprobe. These measurements confirm the existence of a silicon- and aluminum-rich region extending about 10 µm below the surface, and also indicate iron-rich locations which coincide with silicon-rich locations and have spacings on the order of the grain size. The distribution of aluminum is relatively flat.

The implication of these observations in terms of possible origins of the chemical inhomogeneities is discussed briefly.